Embodiments described herein generally relate to phase locked loops (PLLs), and more particularly embodiments described herein relate to PLLs having phase frequency detectors (PFDs) with low noise.
Unless otherwise indicated herein, the circuit and the circuit methods described in the background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the background section.
A PLL is a control circuit configured to generate an output signal where a phase of the output signal is either in phase with a reference signal or otherwise has a fixed phase relationship with the reference signal. The reference signal is typically a clock signal, which may be generated by a crystal oscillator or the like.
In general a PLL includes a phase detector and a voltage controlled oscillator (VCO), which generates an output signal of the PLL. The phase detector is configured to detect a phase difference between the reference signal and a feedback signal (feedback of the output signal of the PLL) from the VCO. The phase detector then generates one or more output signals to drive the VCO to increase, decrease, or maintain the frequency of the output signal generated by the VCO so that the phase difference between of the reference signal and the feedback signal is zero or some other constant value, i.e., matched phase. That is, the output signals of the phase detector are used to control the VCO via a feedback loop.
Because the phase of the output signal generated by the VCO is matched with the phase of the reference signal, PLLs may be used in a variety of applications (e.g., signal demodulation) to track an input signal having an input frequency. Further, because of the foregoing described feature, PLLs are often used as frequency synthesizers to generate an output signal having a frequency that is a multiple of the reference signal. A synthesized signal may have a frequency that is a fraction of the frequency of the reference signal or may be many gigahertz.
PLLs have found wide spread use in a number of fields including wireless communications (e.g., radio frequency communications), telecommunications, computer networking, and other electronic applications. While PLLs have found wide spread use in a number of fields, PLL manufacturers continue to strive to develop improved PLLs having reduced noise and improved efficiency.